telemetry: Allow page tests to be configured via constructor

tools/perf/measurements/screenshot.py

Index: tools/perf/measurements/screenshot.py
diff --git a/tools/perf/measurements/screenshot.py b/tools/perf/measurements/screenshot.py
index 8564c8a473236a06435c33aaef30ee18a4521a9e..cc15ac13ce4848470ee265d33d0613b1ed528b28 100644
--- a/tools/perf/measurements/screenshot.py
+++ b/tools/perf/measurements/screenshot.py
@@ -13,17 +13,16 @@ class Screenshot(page_test.PageTest):
     super(Screenshot, self).__init__(
         action_name_to_run = 'RunPageInteractions',
         is_action_name_to_run_optional=True)
+    self._png_outdir = None
 
-  @classmethod
-  def AddCommandLineArgs(cls, parser):
+  def AddCommandLineArgs(self, parser):
     parser.add_option('--png-outdir',
                       help='Output directory for the PNG files')
 
-  @classmethod
-  def ProcessCommandLineArgs(cls, parser, args):
+  def ProcessCommandLineArgs(self, parser, args):
     if not args.png_outdir:
       parser.error('Please specify --png-outdir')
-    cls._png_outdir = args.png_outdir
+    self._png_outdir = args.png_outdir
 
   def ValidateAndMeasurePage(self, page, tab, results):
     if not tab.screenshot_supported: